1. Field of the Invention
The present invention relates to a process for the preparation of an aqueous polymer dispersion by polymerizing monomers having at least one vinyl group by the free radical aqueous emulsion polymerization method, in which an amphiphilic substance is added to the polymerization vessel before and/or during the polymerization.
2. Description of the Background
Aqueous polymer dispersions (latices) are generally known. They are fluid systems which contain, as the disperse phase in an aqueous dispersing medium, polymer coils (i.e. polymer particles) consisting of a plurality of intertwined polymer chains.
The diameter of the polymer particles is frequently from 10 to 2000 nm.
As in the case of polymer solutions on evaporation of the solvent, aqueous polymer dispersions have the potential to form polymer films on evaporation of the aqueous dispersing medium, and they are therefore used in particular as binders, for example for paints or for materials for coating leather, paper or plastics films. They are becoming increasingly important owing to their environmentally friendly properties.
An important feature of aqueous polymer dispersions is the diameter of the polymer particles present as the disperse phase, since the size of the polymer particles or their size distribution plays a role in determining a number of performance characteristics of aqueous polymer dispersions. For example, films of finely divided aqueous polymer dispersions have high gloss (cf. for example, Progress in Organic Coatings 6 (1978), 22). Furthermore, the power of finely divided aqueous polymer dispersions to penetrate into porous but relatively dense substrates, such as paper, leather or a render surface is greater than that of coarse-particled aqueous polymer dispersions (for example, Dispersionen synth. Hochpolymerer, Part II, Anwendung, H. Reinhard, Springer-Verlag, Berlin (1969), page 4).
On the other hand, coarse-particled aqueous polymer dispersions have, for example, lower flow resistance than finely divided aqueous polymer dispersions, the composition and solids concentration otherwise being identical (for example, Dispersionen synth. Hochpolymerer, Teil II, Anwendung, H. Reinhard, Springer-Verlag, Berlin (1969), page 5). Aqueous polymer dispersions whose polymer particle diameters are distributed over a relatively large diameter range also have advantageous flow behavior (cf. for example, DE-A 42 13 965).
Establishing the diameters of the dispersed polymer particles in a controlled, reproducible manner tailored to the particular intended use is therefore of key importance in the preparation of an aqueous polymer dispersion.
The most important method for the preparation of aqueous polymer dispersions is the free radical emulsion polymerization method, in particular the free radical aqueous emulsion polymerization method.
In the latter method, monomers having at least one vinyl group are usually subjected to free radical polymerization under the action of free radical polymerization initiators dissolved in the aqueous medium, to give polymer particles present directly as the disperse phase in the aqueous dispersing medium. The aqueous polymer dispersions prepared by the free radical aqueous emulsion polymerization method are usually referred to as aqueous primary dispersions, in order to distinguish them from the aqueous secondary dispersions. In the case of the latter, the polymerization is carried out in a nonaqueous medium. Dispersing in the aqueous medium is not effected until after the polymerization reaction is complete.
The monomers to be polymerized are distributed in the form of droplets (the droplet diameter is frequently from 2 to 10 .mu.m) in the aqueous medium with formation of an aqueous monomer emulsion. However, these monomer droplets are not the sites of the polymerization but act merely as a monomer reservoir. Rather, the polymerization sites are formed in the aqueous phase, which always contains a limited amount of the monomers to be polymerized and the free radical polymerization initiator in dissolved form. Chemical reaction of these reactants present in solution results in the formation of oligomer radicals, which are precipitated as primary particles above a critical chain length (homogeneous nucleation). The formation of primary particles presumably takes place up to the point at which the rate of formation of the free radicals in the aqueous phase is equal to the rate of their disappearance due to free radical capture by polymer particles already formed. This polymer particle formation phase is then followed by the polymer particle growth phase, i.e. the monomers to be polymerized diffuse from the monomer droplets acting as a reservoir, via the aqueous phase, to the primary particles formed (whose number and surface area are very much greater than those of the monomer droplets), in order to be incorporated into said primary particles by polymerization (cf. for example, Faserforschung und Textiltechnik 28 (1977), Part 7, Zeitschrift fur Polymerforschung, page 309). By controlled addition of suitable dispersants, both the disperse phase of the monomer droplets and the disperse phase of the polymer particles formed are, if required, stabilized.
While the process of polymer particle growth usually takes place systematically, the polymer particle formation is essentially a stochastic process, i.e. the number of primary polymer particles formed and hence the diameters of the final polymer particles resulting after the end of the polymerization fluctuate from polymerization batch to polymerization batch. The product quality fluctuates in a corresponding manner (identical reproduction is usually not possible). This applies very particularly in the case of a high solids volume content (.gtoreq.50 Vol.-%) of the aqueous polymer dispersion, since, for example, the viscosity of highly concentrated aqueous polymer dispersions is particularly sensitive to the number and size of the polymer particles contained in dispersed form.
It is known that a controlled free radical aqueous emulsion polymerization procedure is possible by initiating it in the presence of a surfactant dissolved in the aqueous medium, the surfactant content of the aqueous medium being such that it is above the critical micelle formation concentration of said medium (cf. for example, High Polymers, Vol. IX, Emulsion Polymerization, Interscience Publishers, Inc., New York, Third Printing, 1965, page 1 et seq.).
The term surfactant means amphiphilic substances which, on dissolution in water, are capable of reducing the surface tension a of pure water significantly (as a rule by at least 25%, based on the a value of pure water) before reaching the critical micelle formation concentration.
The term "amphiphilic" indicates that surfactants have both hydrophilic and hydrophobic groups. Hydrophilic groups are those which are drawn into the aqueous phase, whereas hydrophobic groups are forced out of the aqueous phase.
In highly dilute aqueous solutions, surfactants are therefore present essentially independent molecules in solution, their amphiphilic structure resulting in accumulation at the water surface with oriented adsorption, which reduces the surface tension.
In concentrated aqueous solutions, on the other hand, surfactants are present predominantly as micelles in solution, i.e. the surfactant molecules are arranged in the aqueous solution predominantly in a state of relatively high aggregation, i.e. as micelles, in which they are oriented in such a way that the hydrophilic groups face the aqueous phase and the hydrophobic groups point toward the interior of the micelle. As the surfactant concentration increases further, essentially only the number of micelles per unit volume increases, but not the number of surfactant molecules dissolved in molecular form per unit volume.
The transition from the aqueous molecular solution to the aqueous micellar solution usually takes place relatively abruptly, as a function of the surfactant concentration, which is evident from correspondingly abrupt changes in the concentration dependence of many macroscopic properties (for example the surface tension) and defines the critical micelle formation concentration (usually stated as molar concentration c.m.c.) (inflexional point in the concentration dependence of the property). At concentrations above the critical micelle formation concentration, the term micellar solutions is used. Here, the term solution is intended to express the fact that the visual appearance of a micellar aqueous surfactant solution, like that of a molecular aqueous surfactant solution, is the same as that of a clear aqueous solution. The relative molecular weight of surfactants is usually &lt;2000, and there is usually a rapid exchange (a dynamic equilibrium) in their micellar aqueous solutions between the various surfactant fractions present in solution in molecular and micellar form.
The following are typical examples of surfactants (source: Ull-manns Encyclopadie der technischen Chemie, Verlag Chemie, 4th edition, Vol. 22, page 456 et seq.):
a) Perfluorononanecarboxylic acid (c.m.c. at 20.degree. C. and 1 atm in water=10.sup.-5 mol/l; .sigma. of the substituted aqueous solution=20 mN/m); PA0 b) Sodium 1-decyl sulfate (c.m.c. at 20.degree. C. and 1 atm in water=3.4.multidot.10.sup.-2 mol/l; .sigma. of the associated aqueous solution=40 mN/m).
The surface tension of pure water at 20.degree. C. and 1 atm is 73 mN/m.
It is now generally assumed that the surfactant micelles present in an aqueous medium are nucleation centres for the formation of primary polymer particles (the term micellar nucleation is also used). If the free radical aqueous emulsion polymerization is initiated, for example, in the presence of a large number of surfactant micelles, many small final polymer particles are obtained, whereas initiation in the presence of a small number of surfactant micelles gives a few large polymer particles. At the same time, the surfactant generally reduces both the polymer particle/aqueous medium interfacial tension and the monomer droplet/aqueous medium interfacial tension and is thus capable of stabilizing the particular disperse phase as a dispersant, which has an advantageous effect on the free radical aqueous emulsion polymerization. On the other hand, the decrease in surface tension is usually disadvantageous, said decrease being caused by the surfactant and increasing the tendency to foam formation.
While the validity of the abovementioned relationships may be satisfactory qualitatively (smaller polymer particles are obtained with increasing amount of surfactant, and vice versa; cf. Dispersionen synthetischer Hochpolymerer, Part I, F. Holscher, Springer-Verlag, Berlin (1969), page 81), the quantitative relationship is as a rule just as unsatisfactory as the reproducibility.
EP-B 40 419 (e.g. page 5, line 16 et seq. and Example 1), DE-A 23 21 835 (e.g. page 14, line 9 et seq.) and Encyclopedia of Polymer Science and Technology, Vol. 5, John Wiley & Sons Inc., New York (1966), page 847, therefore recommend that, for establishing the final polymer particle size in a controlled manner, the polymer particle formation phase should be separated from the polymer particle growth, i.e. a defined amount of a separately preformed aqueous polymer dispersion (nuclear or seed polymer dispersion) is added, for example before the beginning of the free radical aqueous emulsion polymerization, and the polymer particles contained in this seed are allowed only to grow in the course of the actual free radical aqueous emulsion polymerization. The diameter of the seed polymer particles and the ratio of initially taken seed polymer particles and monomers to be polymerized essentially determine the size of the final polymer particles in the resulting aqueous polymer dispersion. The more finely divided the seed and the greater the amount of seed used, the smaller are the resulting final polymer particles for a given amount of monomer. If a broad distribution of the diameters of the polymer particles is desired, additional seed polymer dispersion is added to the polymerization vessel also during the free radical aqueous emulsion polymerization of the monomers. In this way, the resulting aqueous polymer dispersion comprises various generations of seed polymer particles grown to different final sizes. A similar effect can also be obtained by initiating the formation of new micelles in the course of the free radical aqueous emulsion polymerization of the monomers by adding a larger amount of surfactant.
However, the disadvantage of the free radical aqueous emulsion polymerization method with the addition of an aqueous seed polymer dispersion is that the aqueous seed polymer dispersion has to be stored prior to its use, which frequently entails problems owing to the basic sensitivity of aqueous polymer dispersions (the subsequent attempt to reduce their interface) to frost, shearing, superficial drying and vibration. Moreover, identical product preparation at different production locations requires the corresponding identical availability of such an aqueous seed polymer dispersion. An additional problem is the production of an aqueous seed polymer dispersion in a reproducible manner.